A field effect transistor has been widely used as a unit electronic device of a semiconductor memory integrated circuit, a high-frequency signal amplifier device, a liquid crystal drive device, and the like. The field effect transistor is an electronic device that is most widely put to practical use at present.
In recent years, development of displays (display apparatuses) has rapidly progressed, and a thin film transistor (TFT) has been widely used for a display (e.g., liquid crystal display (LCD), electroluminescence (EL) display, or field emission display (FED)) as a switching device for applying a drive voltage to the display element.
A transparent semiconductor thin film formed of a metal oxide (particularly zinc oxide crystals) that exhibits excellent stability as compared with a silicon-based semiconductor thin film has attracted attention in recent years.
For example, Patent Documents 1 and 2 disclose a method that crystallizes zinc oxide at a high temperature to form a thin film transistor. Patent Document 3 states that a thin film transistor that exhibits high mobility was obtained by patterning an amorphous oxide film, and crystallizing the patterned amorphous oxide film.
Patent Document 4 discloses a thin film transistor that utilizes In2O3, and states that it is desirable that indium oxide be amorphous before performing a heat treatment, and be crystalline after performing the heat treatment. However, the thin film transistor disclosed in Patent Document 4 that utilizes In2O3 crystals as the channel layer has normally-on properties, and an increase in carrier concentration and a decrease in mobility may occur due to the crystallization step.